Solar cells are the main component of a solar photovoltaic system that converts the energy of sunlight directly into electrical energy, and can be prepared by using monocrystalline, polycrystalline or amorphous silicon-based semi-conductors.
Several or dozens of solar cells are usually interconnected in series or parallel and packaged to form a solar cell module with other units to protect the cells for a long period.
Recently, in consideration of environmental or energy problems, solar cells are becoming more important and extensive research and development have been carried out. A typical solar cell module, as shown in FIG. 1, is composed of a glass substrate (11) as a protective transparent unit on the front, a back protective film (14) as a protective unit on the back, and a solar cell (13) encapsulated by two encapsulant sheets (12a, 12b). Such solar cell module is prepared by sequentially laminating a glass substrate (11), an encapsulant sheet (12a), a solar cell (13), an encapsulant sheet (12b), and a back protective film (14), followed by heating and pressing the stacked components to harden the encapsulant sheets (12a, 12b) by cross-linking so that an integrated module can be obtained.
The encapsulation process of the solar cell module is conducted by heating the stacked components to melt the encapsulant sheets, applying pressure and degassing. In the process, the encapsulant sheets containing cross-linking agents are subjected to a cross-linking reaction by heat so that the solar cell is combined between the glass substrate and the back protective film to obtain a solar cell module.
Generally, a transparent soft resin sheet, e.g., an ethylene-vinyl acetate (EVA) copolymer, comprising cross-linking agents is used as an encapsulant sheet for solar cell modules.
However, conventional methods have the problem that a solar cell module may crack by applied pressure during the process for adhering encapsulant sheets to solar cells. Further, when cross-linking reactions are carried out under insufficient degas levels, air bubbles can be generated in the solar cell module, leading to a lower generation efficiency and deterioration of reliability. In order to solve such problems, Japanese Patent Laid-open Publication No. 2000-183388 discloses an encapsulant sheet for solar cell modules comprising continuous embossed parts having a height of 15 to 50 μm formed on the surface. The embossed parts are formed to improve fusion-adhesive or pressure-adhesive properties during a lamination process and to cushion solar cells when they are combined with encapsulant sheets.
Further, Japanese Patent Laid-open Publication No. 2002-185027 discloses an encapsulant sheet having a plurality of concave and/or convex parts formed on the surface by embossing. Particularly, it is disclosed that the convex parts formed on the encapsulant sheet may have a hemispherical shape having a diameter ranging from 0.1 to 2 mm and a height ranging from 20 to 95% of the thickness of the film and the total area of the convex parts is preferably 1 to 50% of the film area.
However, such encapsulant sheets are still insufficient to prevent solar cell damage and air bubble generation, and therefore, more improved encapsulant sheets are required.